1. Field of the Invention
The present invention relates to a method and equipment for picking up antenna radiation patterns in near fields.
It is known that the field radiated over a great distance r by an antenna may be expressed in the following form: ##EQU1## where r, .theta. and .PHI. represent the spherical coordinates of a point in space, .lambda. the wavelength of the radiation emitted and K a constant. The antenna radiation properties are characterized in amplitude and phase by the function A (.theta., .PHI.).
Antenna radiation patterns are generally used to indicate the curves representing the spatial distribution of the radiated energy. When the power emitted by the antenna is 1 watt, the radiation pattern is depicted by the function [A (.theta.)].sup.2 for a given value of .PHI. or [A (.PHI.)].sup.2 for a given value of .theta..
As an antenna displays the same directivity properties whether used for transmission or reception, these radiation patterns can be determined experimentally by means of a set-up comprising a transmit antenna generating a radiation wave with spherical wave-fronts, an antenna under test working in the receive mode and receiving the electromagnetic field from the transmit antenna and a device for positioning the antenna under test in this field. Provided that the transmit antenna is located sufficiently far away from the antenna under test, then equation (1) gives a good approximation of the radiated field since the spherical wave fronts can be considered as being planar in the vicinity of the antenna under test.
It is generally accepted that this condition is satisfied when: EQU r.gtoreq.2L.sup.2 /.lambda.
where:
r is the distance between the two antennae, PA1 L is the biggest dimension of the antenna under test, and PA1 .lambda. is the wavelength. PA1 (E(.theta..sub.i, .phi..sub.j).
For the larger antennae, this means using base-lines of considerable dimensions for measurement purposes. By way of an example, a 10 GHz antenna 3 meters high must be checked out over a measurement base-line at least 600 meters long.
2. Description of the Prior Art
Over recent years, a certain number of new measurement methods have been put forward to determine the radiation patterns under near-field conditions (r&lt;2L.sup.2 /.lambda.). These methods have brought into play measurement equipment implementing antennae emitting planar waves in the near field, or calcuating means determining an antenna pattern taking into account the sphericity of the waves received by the antenna for which the pattern is being sought.
It has been proposed by the company Scientific Atlanta that direct readings be taken using a transmitting antenna with greater dimensions than the antenna under test. The transmit antenna is of the horn-type placed at the focal point of a large parabolic reflector and throwing radiation thereon. In this instance, the emitted wave is planar and homogeneous at the antenna under test; it were as if the two antennae were very far apart.
The difficulty in this method lies in the manufacture of the transmitting antenna and notably the reflector thereof, in view of the degree of accuracy required. As things stand, this method is costly and does not permit measurement readings for antennae with dimensions in excess of 1.2 meters.
In French patent application FR-A-2434497, the company Plessey describes a variation on this solution in which the large parabolic reflector of the transmitting antenna is replaced by a large lens made of a light-weight polyurethane foam type dielectric material.
This lens is, it would seem, less expensive than the Scientific Atlanta reflector; it does, however, produce a less homogeneous planar wave at the antenna under test and the working pass band is certainly smaller on account of the need to use absorbent linings to prevent reflections on the lens faces. Furthermore, the limits on dimensions are at least restrictive. For an antenna under test with dimensions of 1.2 meters, use must be made of a lens 2 meters in diameter and more than 1.2 meters thick.
Additionally, according to former art, reading can be taken in the near field and be followed by calculations intended for correcting the effects due to curvature of the emitted wavefront at the antenna under test. The detection of a radiation pattern is carried out in two stages that can be recapped in relation to FIGS. 1 and 2.
First stage
Radio-frequency transmitter 1 feeds antenna 2 the radiation from which, in the form of spherical wave-fronts, is received by antenna 3 under test set up at a distance R from antenna 2. Antenna 3 is mounted on a positioner 4 that is controlled both is azimuth .phi. and elevation .theta. by computer 5. The signal received by antenna 3 in its successive positions is detected in amplitude and phase by coherent detector 6. Detector 6 is connected to computer 5 and the latter thus receives signals representing the values of the field E (R, .theta., .phi.) recorded in terms of the two angles .theta. and .phi. for a given value of R. The measurements are recorded in digital form and are stored in a two-dimensional table:
Second stage
Calculation of the radiation patterns corrected for near-field effects occurs in the second stage.
It is temporarily assumed that antenna 3 serves as the emitter and antenna 2 serves as the receiver. Transmitter antenna 3 is assumed to be fixed and receiving antenna 2 moves over a sphere with an origin O in azimuth and elevation coordinates.
If the electromagnetic field as radiated at all points on any closed surface around the antenna is known, then it is possible to calcuate the field at any point in space outside this surface using Kirchhoff's equation: ##EQU2## (Cf. Principles of Optics by Max Born and Emil Wolf, Pergamon Press, 1959, page 376, equation (7)), a similar formula (formula (5)) is given by V. I. TURCHIN et al. in "Antenna Testing Based on Near Field Measurements" Radio Engineering and Electronic Physics, Vol. 24 (1979) Dec., No. 12, Silver Spring, Md., USA, where S is the surface area around the antenna, r and r' are the vectors defining the positions of the points located respectively outside and on the surface S and n is the unit vector normal to S at r' ##EQU3##
If S.sub.1 is that part of the surface S seen from the point r, when the surface S.sub.1 is sufficiently far away from the antenna for the fields on S.sub.1 to be represented by a geometrically optical approximation, then Kirchhoff's equation can be reduced to a more simplified form: ##EQU4##
When the surface S is a sphere centered on O and having a radius R and S.sub.1 (FIG. 2) is half such sphere and for points r a long way from the antenna (r&gt;&gt;R), Kirchhoff's equation may be written as: ##EQU5## where .theta. and .phi. are the angles of elevation and azimuth defining a point on the sphere (hence dS=R.sup.2 sin .theta.d.theta.d.phi., and u is the unit vector of r).
Although equation (4) has been formuated under the assumption that 3 and 2 are the transmitting and receiving antennae respectively and that the former is fixed and the latter movable throughout a sphere of radius R, the same equation holds true when 3 and 2 are respectively receiving and transmitting antennae where the former is movable about a point O and the latter is fixed.
It can be seen that the method employed in former art as just described includes the calcuation by computer of a bidimensional integral transform. This makes it possible to ascertain radiation patterns for very large antennae. However, the greater the antenna, the greater the volume of computer memory space and the longer the computation time. This method is particulaly penalizing when it comes to determining a single pattern of the type .vertline.A (.theta.).vertline..sup.2 for a set value of .PHI. or .vertline.A (.phi.).vertline..sup.2 for a set value of .theta.. This is because to calculate the corrections, a determination is required of all the values of the two-dimensional function E (.theta., .phi.).